1. Field of the Invention
The present invention relates to a technology for printing.
2. Description of the Related Art
A commonly-used image forming apparatus, including a copier and a printer, that is capable of printing on both sides of a transfer medium, such as paper, uses interleaf printing. The interleaf printing is a method of forming an image on a second side of the nth sheet and on a first side of the (n+2)th sheet alternately. For example, images are formed (printed) on recording sheets in the order of a first side of the first sheet, a first side of the second sheet, a second side of the first sheet, a first side of the third sheet, a second side of the second sheet, a first side of the forth sheet, and so on. In the Interleaf printing, a recording sheet is fed from a feed tray specified by a user upon printing its first side, and temporarily kept in a double-sided printing conveying path after the first side has been printed. Upon printing its second side, the recording sheet (whose first side has already been printed) is fed from the conveying path, where the sheet has been kept temporarily, printed with an image, and ejected from the image forming apparatus. Japanese Patent Application Laid-open No. 2005-084547 discloses an example of the double-sided image forming apparatus that forms an image using the interleaf printing method.
However, in the interleaf printing method, the (n+1)th recording sheet can be fed from the feed tray to have its first side printed, and conveyed to the conveying path, without a convey command being issued for the nth sheet in the conveying path, having its first side printed and waiting to have its second side printed. The convey command can be withheld due to delay in an image expanding process in a controller, for example, and cause collision of the nth recording sheet and the (n+1)th sheet, further causing a paper jam. Such collision can cause damage to rollers or gears conveying the recording sheets. Furthermore, the user is required to remove the collided sheets from the conveying path.